The invention relates to viscosity meters of the type having a probe consisting of a straight needle which is adapted to be partially immersed in the fluid whose viscosity is to be measured. An intermediate portion of the needle is secured against movement to constitute a node and a portion of the needle remote from the immersed portion is provided with drive means for vibrating the needle transversally and with pick up means for delivering a signal representative of the amplitude of the vibration.
Viscosity meters of the above-defined type have been known for score of years (French No. 899,057). However, it was found that they are not suitable for use when a high accuracy is required or/and under circumstances where the viscosity may vary in a substantial range; consequently U-shaped probes rather than needles are used in the present day mechanical vibration viscosity meters (French No. 2,353,847). On the other hand, the increased complexity of the viscosity meters using a U-shaped probe represents a definite drawback.
The inventors have now found that the main deficiencies of the prior art needle type viscosity meters may in fact be removed if they are driven at the natural or resonance frequency of the needle; a difficulty then occurs, which is the variation of that frequency if the length of the immersed portion or the viscosity changes.
It is an object of the invention to provide a viscosity meter which retains the simplicity in design associated with a needle probe and substantially improves upon the accuracy, ease of use and range of operation of the prior art viscosity meters. It is an ancillary object to provide a viscosity meter whose response may be rendered substantially independent of the temperature variation in a broad range.
For that purpose, the needle is arranged to be vibrated by a stationary drive coil fed with an A.C. energizing current by an electric circuit connected to receive a signal from the pick up means at the vibrating frequency and including a feedback loop arranged to adjust the frequency of the A.C current at the natural frequency of the needle. The driving coil cooperates with a ferromagnetic element, typically a permanent magnet, carried by the needle.
Experience has shown that the response of a viscosity meter according to the invention is such that it is suitable for use in a very wide viscosity range, typically of from 1 to 10 cpo. An ancillary advantage is a life duration much increased as compared with the systems in which a probe is oscillated by a rotating cam and follower mechanism.
The needle will typically be secured in an end wall of a tube section whose opposite end is secured to a stationary plate. Then the vibration node will usually be between the ends of the tube section.
A measure of the viscosity of the fluid may be the magnitude of an electric signal delivered by the pick up means when a predetermined electrical power is applied to the drive coil. There exists pick up means (e.g. Hall elements) which provide a voltage which is in direct relation to the amount of vibration. In another embodiment, the electrical current applied to the drive means is adjusted for maintaining the amplitude of the output from the pick up means at a constant value and the power is measured for providing an indication of the viscosity.
The output from the pick up means, as well as the movement of the needle is approximately sine shaped. It is however preferable to deliver a square wave signal to the coil. It can be generated by amplifying and clipping the signal delivered by the pick up. With that approach, the vibration starts without an external action when power is applied.